The present invention relates to a method for producing a value T being representative of the mean engine torque generated on a crankshaft of an internal combustion engine. It also relates to a device for producing such a value.
For engine diagnostic purposes, a useful parameter is the engine torque generated on the crankshaft of an engine. Faults or irregularities in the function of the engine will result in diminished or irregular engine torque. In a more detailed diagnosis, once a faulty engine torque value has been detected, it could be further analysed so as to provide information regarding the likely source of error, such as an injector fault or other error source. Thus, it is desirable to provide a measure of the engine torque generated on a crankshaft of an engine in a vehicle. Previously, a number of methods have been proposed for determining the engine torque from selected measurements performed on the engine and using different mathematical methods.
U.S. Pat. No. 5,771,483, Moine et al. describes one such prior art method for calculating the torque of an engine, including the detection of the passage before a sensor of each of the teeth of the inertial flywheel of an engine, and using the time of passage of each of these teeth in the calculation of a torque of an engine. Another method of this type is described in EP 1 052 488, Abida et al.
It is desirable to provide a measure of the mean engine torque generated on a crankshaft of an engine in a vehicle that is useful for engine diagnostic purposes. It is desirable to provide a measure of the mean engine torque that may be used for diagnostics of an engine during normal operating conditions. It is desirable to provide a measure of the mean engine torque that may give a sufficiently accurate measure whilst being limited to reasonable requirements for processor capacity, storage space etc.
According to an aspect of the present invention, a method for producing a value T being representative of the mean engine torque generated on a crankshaft of an internal combustion engine is provided, the method comprising the steps of
producing a speed vector including values being representative of instantaneous speeds of the engine during a sampling period,
determining a mean engine speed n during said sampling period from the speed vector
determining a value Px being representative of the frequency contribution of the speed vector at the frequency x*n, where x is a pre-selected order, and
producing a value T being representative of the mean engine torque on the crankshaft during said sampling period wherein T is derived from a mathematical expression including a polynomial having at least a term k*n*Px, wherein k is a polynomial constant.
The method according to the invention provides a useful measure of the mean engine torque that has the advantage of being applicable to situations in which the vehicle is under load, i.e. during normal driving conditions. It has further the advantage that it is executable using a relatively low number of samples and using relatively few calculations, resulting in a fast calculation that may be made without need for additional processing power in the vehicle.
The mean engine torque obtained using the method may be calculated at regular intervals during use of the vehicle and logged to provide a stored log being a chart over the vehicle's behaviour over time. When the vehicle is serviced, the log may be studied and conclusions regarding the state of the engine and possible necessary repair or replacement work.
A further advantage is that the mean engine torque measure obtained may be used to objectively evaluate the function of the engine. An ideal engine torque measure may be set for certain engine conditions and comparing the calculated engine torque with the ideal torque give an indication whether the engine fulfils the desired requirements or not.
The internal combustion engine may be used in different applications, such as in vessels or vehicles. Vehicles include e.g. cars, heavy duty vehicles and tracked vehicles. Preferably, the expression includes the polynomial: k0+k1*Px+k2*n+k3*n*Px, wherein kθ, k1, k2 and k3 are polynomial constants, and advantageously T=k0+k1*Px+k2*n+k3*n*Px.
Advantageously, the selected order x may be equal to the number of cylinders of the engine divided by two. This selection of the order has been found to reflect the effect of a four-stroke engine particularly well. For two-stroke engines, the selected order x may instead be equal to the number of cylinders.
Advantageously, the sampling period corresponds to at least 10 rotations of the engine. Such a sampling period is long enough to ensure that the mean engine torque value includes the effect of all of the cylinders of the engine.
Advantageously, the speed vector comprises values t being representative of instantaneous speeds of the engine being the passages of time between subsequent rotational indexing references arranged in connection with a flywheel or crankshaft of the engine. This embodiment provides an efficient and relatively reliable method for calculating values being representative of instantaneous engine speeds.
In particular when a flywheel is used, the arrangement of the rotational indexing references is irregular at some locations. In this case, the method may comprise a compensation step when generating the speed vector, wherein effects of any irregularities in the arrangement of the rotational indexing references are compensated for.
Preferably, the value Px representing the power contribution for the frequency x*n is determined using a Fourier series analysis of the speed vector. In this case, the speed vector should preferably include at least 500 samples so as to give satisfactory resolution.
Alternatively, the value Px may be determined by using a Fast Fourier Transform for the phase-amplitude transformation of the speed vector.
In this case, the RMS value of the area under the peak at the frequency n*x in the phase-amplitude transformation is used as Px.
If Fast Fourier Transform is used in combination with selecting the RMS value of the xth order peak as Px, it has been found that good results are obtained when the speed vector includes at least 1000 samples, preferably at least 2000 samples.
Regardless of the phase-amplitude transform method used, the values t may advantageously be sampled with a sample interval with a duration of less than or equal to 10 microseconds, preferably less than or equal to 1 microsecond.
The loading conditions during the sampling period may be selected so as to correspond to at least 50% load, preferably to at least 60% load. This is advantageous since the conditions correspond to normal conditions when a vehicle such as a truck is in use.
Further, the engine speeds during the sampling period may be selected to be at least 1000 rpm. For cars, higher engine speeds are used, and the engine speeds during the sampling period may be selected to be at least 1500 rpm.
Advantageously, a value T produced according to the invention may be compared to reference value Tref being representative of the engine torque of a standard engine, to enable evaluation of the engine for which the value T was determined.
In a second aspect of the invention, a device for the production of a value T which is representative of the mean engine torque generated on a crankshaft of an internal combustion engine, comprising
a measuring device for measuring instantaneous speeds of the engine during a sampling period,
calculation means for producing a speed vector of values being representative of the instantaneous speeds of the engine measured during said sampling period,
calculation means to derive a mean engine speed n during said sampling time using the speed vector,
calculation means to derive a value Px being representative of the power contribution for the frequency x*n from the speed vector, where x is a pre-selected order, and
calculation means to derive the desired value T being representative of the mean engine torque on the crankshaft during said sampling period wherein T is derived from a mathematical expression including a polynomial having at least a term k*n*Px, wherein k is a polynomial constant, stored in a memory.
A device according to the invention provides the same advantages and may be combined with especially advantageous features in accordance with the method according to the invention as described above.
The calculation means may but must not all be comprised within one and the same calculation unit, such as a processor unit, a microprocessor, an embedded processor or other suitable calculation unit.
Advantageously, the measuring device may comprise a sensor being arranged for sensing the passage of subsequent rotational indexing references arranged in connection to a flywheel or a crankshaft of the engine, said passages of time being the values t of the speed vector being representatives of instantaneous speeds of the engine.
Preferably, the measuring device comprises a calculation means for determining the speed between subsequent rotational indexing references. The calculation means may but need not be comprised in the same calculation unit as the previously mentioned calculation means of the device.